Winter Floods in Britain Connected to Atmospheric Rivers

27 February 2012

The AR that contributed to serious flooding in Northwest England on 19th November 2009. The image uses total column water vapour (in cm) from the ERA-Interim reanalysis.

Click image for more details

A journal article entitled Winter Floods in Britain are Connected to
Atmospheric Rivers by David Lavers, Richard Allan, Eric Wood, Gabriele
Villarini, David Brayshaw, and Andrew Wade appeared in Geophysical Research
Letters in December 2011.

Winter floods have recently been prevalent in the United Kingdom (UK) and
Western Europe. With climate change expected to lead to an enhancement of
extreme rainfall / flooding, this research was undertaken to improve
hydrometeorological process understanding of flooding in UK river basins,
as this is an important step in assessing the future risks of flooding in
the UK and other mid-latitude regions. This work builds on results that
have emerged from NOAA's HMT-West
program, which had identified ARs as
crucial to west coast extreme precipitation and flooding. Combined with
recent publications on the role of AR conditions in Andean precipitation in
Chile (Viale and Nunez, 2011, J. Hydrometeor.), it is clear that the AR
phenomena are important in at least two other continents than solely North
America. The findings reinforce the important HMT research and prototyping
direction focused on improving physical understanding, observations,
modelling, predictions, and climate projections of ARs on the U.S. West
Coast.

The paper shows that winter flooding in Britain is connected to ARs. By
combining river flow and rainfall records, satellite data and climate model
reanalyses, winter flood events in a range of British river basins and the
atmospheric circulation that occurred concurrently with the floods were
analysed. A severe flood in Northwest England in November 2009 was used as
a case study to highlight the link between ARs and extreme flooding (see
Figure). It was both the persistent nature of this particular AR (the AR
was located over the basin for at least one day) and orographic enhancement
of rainfall that resulted in high rainfall totals and flooding.
Furthermore, ARs were shown to occur simultaneously with the 10 largest
winter floods in the study basins indicating that ARs are critical in
explaining winter flooding in Britain. The article also discusses the role
of basin geology in flood generation highlighting how the AR-flood
connection is strongest in basins with a rapid-response to rainfall (i.e.
basins with impermeable bedrock and soils).

Two factors that could affect ARs in a warming climate were discussed.
Firstly, a change in AR frequency is likely to affect the frequency of
extreme winter flood events, and secondly higher atmospheric water vapour
content is expected to lead to ARs delivering more rainfall and a risk of
larger floods. Further research is recommended to determine how ARs may
alter under current projections of climate change.